Easter, a Christian holiday commemorating Jesus Christ’s resurrection from the dead, is defined not only by Church liturgy but also by astronomical circumstance. As specified by the First Council of Nicaea, Easter is not a straightforward anniversary, always recurring on the same date from now until eternity. Instead, it is to be celebrated on the first Sunday after the first full moon after the spring equinox—an occasion already laden with astronomical significance. The equinox, after all, is a day when time is split equally into 12 hours each of light and darkness, of illumination and obscurity.
ShareThe stakes of getting the date right were unusually high, Heilbron writes. If the faithful were to worship Easter on the wrong Sunday, out of sync with the rest of Christendom, then their very souls could be at risk. This was not merely an academic concern: at the height of the Church’s calendar problem, in the second half of the 16th century, the eastern Church and the western Church were an incredible ten days out of sync with one another. This was only reconciled in 1582 when Pope Gregory XIII implemented what has become known as the Gregorian calendar reform.Gregorian reform eliminated, at a stroke and literally overnight, ten entire days from the western European calendar. People going to bed on October 4th, 1582, when the reform was implemented, would have woken up the next morning to find it was October 15th. Although this disorienting reform was intended specifically to put the calendar back on track for reaching the next spring equinox on March 21st, March 21st is not always the true, astronomical spring equinox. To determine exactly when the equinox would be, in the future—and, thus, when Easter should properly be celebrated—a more subtle and astronomically precise tool of measurement was required. A meridian line. (Read more.)